1. Field of the Invention
This invention relates to superconducting quantum interference devices, known as SQUIDs used for measurement of a magnetic flux.
2. Background Art
Superconducting QUantum Interference Devices (SQUIDs) are the most sensitive magnetic field detector known. A SQUID can be configured to measure a minute change of any physical quantity that can be converted to a flux, such as voltage, current or magnetic field. Such devices can have energy resolution capabilities approaching the quantum limit.
There are two kinds of SQUIDs. The first, the dc SQUID, consists of two Josephson Junctions connected in parallel on a superconducting loop. The second, the rf SQUID, involves a single Josephson Junction on a superconducting loop. In both types, the Josephson Junction is formed by a thin insulating barrier or layer between two pieces or sections of superconductor. The insulating barrier's thickness and cross-section are so very much smaller than the dimensions of the complete circuit loop that electron pairs can tunnel from one side of the Junction to the other without transfer of energy. This makes it possible to have current flow in the absence of all applied voltage. Particularly in the dc, two Junction loop, SQUID the current produced by interference oscillates rapidly as one changes the magnetic field.
Typically, a SQUID produces an output voltage which varies in a periodic manner in response to a small input flux. The extreme sensitivity of such a device derives from the fact that the SQUID can resolve a small fraction of .phi..sub.0, the quantum of magnetic flux, while .phi..sub.0 is itself a very small quantity. Both dc and rf SQUIDs are used, and can be used, as sensors in a wide variety of instruments.
With the advent of high temperature superconductors (HTSC), it has generally been accepted that SQUIDs are one of the few most likely applications for these materials. With the critical temperature of HTSC now higher than the boiling point of liquid nitrogen, the HTSC SQUID can be operated in liquid nitrogen, which is both a much less expensive cryogen and one with a much higher heat capacity. This drastically reduces the coolant cost for the device. Such a HTSC SQUID therefore will certainly be more versatile, economic and easy to use.
Unfortunately, realization of this application has been hampered due to a number of unsolved technical challenges.
The first problem is lack of a reliable technique for fabricating the Josephson Junction; this is the essential element of a vast majority of traditional superconducting electronics. A classic Josephson Junction is extremely difficult to fabricate with HTSC. The main reason for this problem is the very small coherence length of the oxide superconductors which is typically on the order of 1 nm. Hence, to fabricate a good Junction it is necessary to have a S-I interface on an atomic scale. The alternative weak link Junction structure, relying on such linear dimensions, is very hard to fabricate even with the most sophisticated lithographic tools available today. Though dc-SQUIDs fabricated with grain boundaries have shown some substantial progress, it is not clear that such a technique can be extended to producing complex circuits.
The second problem concerns the 1/f noise level in HTSC SQUIDs; it is very high compared with that of low temperature SQUIDs. This noise clearly originates in the Josephson elements, and not in the epitaxial HTSC films, for reasons that are not understood.
The third problem is that the characteristic voltage is low, which might be due to the weak tunnelling current. This further lowers the signal-to-noise ratio.
It is difficult to imagine that HTSC SQUIDs will have significant practical applications unless there is a major improvement in their signal-to-noise ratio and sensitivity.
Accordingly, an object of the present invention is to provide a SQUID design, particularly a HTSC SQUID design, which will achieve a high current gain, high signal-to-noise ratio and high sensitivity.
These and other objects of the invention will be apparent to those of ordinary skill in this field from the following description of preferred embodiments.